: Project: 78 Brougham/Transmission Upgrade

06-28-06, 07:12 PM
I was wondering if anybody here has bolted a 700r4 to the 425? If so, what do I need to change or upgrade? I had my 425 entirely overhauled, and it's now putting 230hp to the wheels. I would now like to change the trans/rear and carb. I plan on using this as my weekend cruiser, and would like it to operate as efficient as possible.

06-28-06, 11:15 PM
You'd need a trans adapter, but it should be ok. Remember, there is a lot of torque there, not necessarily a lot of hp. HP is easy to soak up in the trans, but the torque isn't. That is what breaks things.

Advice? Don't put slicks on, and don't allow it to hook, letting the tires spin is a GOOD thing....

Don't put in an OD with the stock 2.28 gears, it will be waaaay to low rpm and the converter will be in stall too much and toast the trans fluid. Ensure that TCC lockup works! This is key to trans life.

And my thoughts are that the 700R4 isn't a good choice for the 425. Remember, with stock cam and heads the engine makes power at LOW rpms, not high, the first gearset in the 700 is a 3.06 and that coupled with a 3.08 or so rear will make it rev up through the rpm range likely faster than the 425 will be very thrilled to do so. So it may end up being slower than the stock THM400 would be even with 2.28's. A THM400 with 3.08's would be a nice running combo.

06-29-06, 12:54 AM
See....That's the kind of info I need. Should I get my TH400 built up to support the power? The only reason I was wondering about the 700R4 was because I usually cruise the highway @ 80+mph and right now I get about 3mpg @ those speeds(no kidding). Would my mileage get better with the 3:08's?

06-29-06, 01:04 AM
3mpg? There is something major wrong then.

My 80 T/A with 70 455, 10:1 compression, Cam Dynamics 284H cam and at 3000 rpm @ 75 mph highway cruise I would get 15 mpg. I had 3.08's and 28in tall tires.

OD will help, with 3.08's, but likely hurt if you have factory 2.28's (common, but not all had them, some had 3.08's). The first gear in the 700R4 isn't gonna be super friendly with the 425. I have it in my LT1 and don't care for it. I still miss my THM400 and my THM350. Just needed OD....

You won't have to build up the THM400 to handle it, it can do that with relative ease. If you are gonna race it, then beef it. If you got the $$ to do it, go for it.

2.73's would a nice gear with the THM400 and 29in tall tires keeping cruise rpms around 2300-2500 rpm @ 80 mph. With the torque of the 425 would do nicely.